This application claims priority from Japanese Patent Applications No. 9-334444 filed Dec. 4, 1997 in Japan, No. 9-337205 filed Dec. 8, 1997 in Japan, No. 9-345242 filed Dec. 15, 1997 in Japan and No. 9-351431 filed Dec. 19, 1997 in Japan, the contents of all four applications being incorporated herewith by reference.
1. Field of the Invention
The present invention relates to Synchronous Transfer Mode (STM) communications networks, and in particular to techniques for transferring data with an Internet Protocol (IP) address by way of an STM connection.
2. Description of Related Art
A characteristic of STM-based circuit-switched networks is that because a signal on a physical channel is multiplexed into time slots and circuit-switched, the delay during communication is extremely short and there is no overhead such as the header required in Asynchronous Transfer Mode (ATM) where virtual paths and cells are employed. In telephones, for example, voice quality problems arise and echo cancelers may be required if there is a delay of more than 20-30 ms. In telephones, video conferencing and other bidirectional services in general, it is preferable for delay to be short. From this point of view, STM is a transport mode which is well suited to digital telephone networks.
The operation of a conventional STM network will be explained with reference to FIG. 1 and FIG. 2. FIG. 1 shows the main components of a conventional STM-based circuit-switched network, while FIG. 2 shows the main components of a conventional local switch.
As shown in FIG. 1, in a conventional STM-based circuit5 switched network, a user operating telephone 11 transmits dialing information 15 (the telephone number of the called party) to source switch 13 in STM network 12 via common channel signaling network 16. This is signaling system No. 7 as defined in ITU-T recommendations. Switch 13 receives the dialing information via software 14 and maps it to an E.164 address (an ISDN address as standardized by the ITU-T) for physical transmission, whereupon STM-based communication is carried out. Namely, source switch 13 establishes an STM connection in STM network 12 and uses this connection to map data from telephone 11 to the E.164 address and transfer it to destination switch 13.
In a conventional STM-based circuit-switched network of this sort, switching is performed by transposing time slots which have been multiplexed into frames, each time slot comprising 8 bytes of data. By way of example, this operation can be implemented by a sequential write random read, as shown in FIG. 2. Namely, time slots can be switched by using sequential counter CTR to write the data at sequential addresses to data buffer memory DBM, and then using address control memory ACM to change the order in which this data is read.
As mentioned above, compared with ATM and the like, the advantages of a conventional STM-based circuit-switched network of this sort are that delay is extremely short and there is no overhead.
However, a conventional STM-based circuit-switched network of this sort requires that data is transferred via an STM connection which has been set up in advance from a source user terminal to a destination user terminal. Its applicability is therefore restricted, and such circuit-switched networks are only being considered for leased data circuits between large businesses. FIG. 3 illustrates a schematic of a dedicated STM connection.
On the other hand , in data communications based on an IP architecture , by writing the IP address, which is the identification number of a terminal, in the header of a packet and sending this to a network such as the Internet which supports IP, each router of a switch or the like simply looks for the immediate transfer destination corresponding to the address. The datagram is successively transferred without requiring a connection to be set up between the two terminals which want to communicate. Thus no dedicated connection is required and data can be transferred by means of an extremely simple procedure. However, under this scheme there is a longer delay than in an STM-based circuit-switched network, and overhead is necessary.